1. Field of the Invention
This invention relates to a fire detector which is capable of detecting changes in physical phenomena caused by fire, for example a change of smoke density, in the analog form, and transmitting a detection signal to a central signal station after conversion of the same into a digital form.
2. Prior Art
A conventional fire detector, for example a photoelectric type fire detector, which is adapted to detect changes in physical phenomena caused by fire, such as a change of smoke density, in the analog form, convert an analog detection signal into a digital signal and transmit the same to a central signal station, includes, as requisites, a light emitting element such as a light emitting diode etc., a photodetector such as a PIN photodiode etc., an amplifier, a sample-and-hold circuit, an A/D converter, and a transmission control circuit. In this type of fire detector, pulse light is radiated to a smoke detection area from the light emitting element when the detector is called by polling from the central signal station and scattered light is incident upon the photodetector and converted into an electric signal. The so converted signal having a level corresponding to the smoke density is amplified and output from the amplifier. The peak level of the amplified output is detected by the sample-and-hold circuit and converted into a digital signal of given bits by the A/D converter. The digital signal is then transmitted to the central signal station, for example, by serial transmission.
However, since the conventional fire detectors need an amplifier, a sample-and-hold circuit and an A/D converter to convert the analog signal into the digital signal and transmit the signal to the central signal station, the circuit arrangements of the fire detectors are somewhat complicated and expensive.
More particularly, the A/D converter has such problems that it becomes very expensive when the number of bits is increased, which hinders digital transmission of the analog detection signal from becoming more accurate for an economical reason and that it requires strict criticality for a reference voltage for the A/D conversion. the sample-and-hold circuit involves a problem that its circuit arrangement must be very complicated because it is required a high impedance.